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The depth to which this conversion occurs depends upon the material used to produce it, and the length of time the process is continued, varying from 1⁄64 inch under the prussiate of potash process to 1⁄16 or 1⁄8 inch in the case of long-continued box case-hardening.

The simplest method of case-hardening is by the prussiate of potash process, for which it is essential that the prussiate of potash be finely powdered, and contain no small lumps.

The piece being heated may then, if small, be dipped in the prussiate of potash, or if large have the same spread upon it.

The prussiate of potash process is, of course, from its expensiveness, both in material and labor, too costly for work to be done in quantities, and box-hardening is therefore resorted to.

Case-hardening.—This is the name given to the process by which the surfaces of articles made of wrought iron are converted into steel, and consists in heating the articles in contact with substances rich in carbon, such as bone-dust, horn shavings, or yellow prussiate of potash.